The photosynthetic activity of algae offers a potentially very productive source of food proteins and other valuable products if practical systems for high density cultivation can be devised. It has heretofore been recognized that yields may be increased and that certain practical problems may be resolved by cultivating herbivore zooplankton, such as brine shrimp for example, in conjunction with the algae. The zooplankton perform the harvesting function and in the process convert the product to high quality animal protein.
The economic viability of such systems is strongly dependent on the population of aquatic animals which can be maintained per unit volume in a liquid medium within cultivation tanks or the like. In the case of brine shrimp (Artemia salina) population densities of about 4,000 animals per liter have been maintained in small tanks with a high liquid flow through rate but such systems are subject to problems which must be resolved to enable practical use on a commercial scale.
For example prior systems of this kind have typically depended on fine mesh screens to confine the animal biomass while enabling a continuous flow through of the liquid medium and nutrients as is required for high density cultivation. Systems relying on screens for this purpose have not been suitable for economical large scale culture as the screens also tend to retain much of the waste materials produced by the animals. Clogging of the screens may occur very rapidly. Trapped waste materials produce a variety of adverse effects including reducing the flow through of liquid, acceleration of ammonia production and depletion of dissolved oxygen.
Materials which can adversely affect a culture and which should be removed are not limited to waste matter produced by confined animals. In the culture of algae, undesirable competitor species or even predator species or organisms may proliferate under certain conditions. Growth of a population of such unwanted organisms tends to be promoted if the organisms are allowed to colonize the inner walls of the cultivation tank. Maintenance of the desired culture of algae may require removal of accumulated substances from the walls of the culture vessel.
Accordingly, effective large scale utilization of algae-zooplankton cultures for the above described purposes requires methods and suitable apparatus for quickly and more efficiently removing waste materials and the like from a confined high density biomass. This should preferably be accomplished on a continuous basis and should not require sizable structural complications of the culture apparatus nor substantial complication of maintenance and operating procedures.
While the problems addressed by the present invention have been discussed above with reference to large scale algae-brine shrimp culture, essentially similar problems from the accumulation of unwanted materials are encountered in some aquariums or other containment systems in which other kinds of aquatic organism are maintained in a liquid environment. The cultivation of oyster larvae for example has been impeded by the above discussed problems inherent in prior techniques and apparatus for the elimination of waste products and the like.
One common prior method for avoiding waste accumulation problems while confining an aquatic animal biomass is the sub-gravel filter as typically employed in small aquariums. In such systems an air-water pump draws water and waste material into a fixed bed of sand or gravel where the material is eventually mineralized to harmless end products. This technique is effective only where the rate of waste generation is relatively small in comparison to that of a high population density system of the kind discussed above. Where higher rates of waste generation are present, known counteractive procedures typically require large and costly filter systems which must be frequently cleaned or changed and which are adversely affected by clogging in the interim. In some prior systems, periodic shifting of the animal population from one container to another may be required which procedure dictates structural complications, adds substantial costs and may adversely affect the health and productivity of the animals.
The present invention is directed to overcoming one or more of the problems as set forth above.